Outboard motors generally comprise a power head consisting of an internal combustion engine of some type standing on end with the crank shaft oriented vertically. The crank shaft is coupled to a drive shaft that extends downwardly supported by a drive shaft housing which also supports a lower unit including a propeller shaft, gears connecting the two shafts and the propeller. The drive shaft housing may also house exhaust ducts which direct exhaust out through the hub of the propeller.
Typically, the power head and drive shaft housing are attached to an adapter plate positioned therebetween. The adapter plate may incorporate means for attaching the motor to a boat and typically has passages therethrough for cooling water and engine exhaust.
Historically, the great majority of outboard motor power heads have been two cycle engines because of their high power to weight ratios. For many years, these two cycle engines were lubricated by mixing oil with the fuel for the engine prior to its induction into the power head. More recently, various other means of lubricating two cycle outboard engines have been introduced which require an oil reservoir separate from the fuel tank. Also, interest in four cycle power heads for outboard motors has greatly increased in recent years.
Lubricating systems for four cycle engine differ from those for two cycle engines due to the fact that the crank case of the four cycle engine is not pressurized during operation of the engine as is the crank case of the two cycle engine. It is therefor possible to utilize a lubricating system wherein the oil is delivered to the bearing surfaces of the engine and allowed to drain by gravity down to the bottom of the engine where it may be collected and pumped to a holding tank as in a dry sump engine; or, directed to a tank in the drive shaft housing.
The invention is directed to the latter arrangement where oil from the power head is permitted to drain by gravity, through passages provided, into an oil tank located in the drive shaft housing.
The use of this arrangement poses a danger of oil flowing in the reverse direction back into the power head when the outboard motor is placed in the horizontal position, as it may be when it is not mounted on a boat. The danger lies in the possibility that oil may find its way into the cylinder head and cause hydraulic lock and possible damage when an attempt is made to start the motor in this condition.
It is the primary purpose of the invention to overcome this problem by preventing the undesirable flow of oil from the oil sump in the drive shaft housing back into the power head.